This invention relates to the use of scleroglucan as a high temperature, viscosity-enhancing additive for cement slurries. It relates to any operation involving a cement slurry having a high viscosity at high temperatures (temperatures typically higher than 90.degree. C.). It is applicable to the oil industry for the cementing of wells for the recovery of a gaseous and/or liquid fluid from a porous subterranean formation. It is also applicable to the cementing of geothermal wells.
More particularly, it is applicable to the placing of the cement in the annular space between the walls of a well and the casing. This imparts improved mechanical strength to the casing, on the one hand, and a perfect fluid-tight seal between the various zones of the porous formation, on the other hand. The placing of the cement is generally performed in the following manner: The cement is poured into the casing at the head of well and is then forced to the bottom so that it can be transferred to and rise into the annular space while flushing out a drilling fluid and optionally a slug fluid. The success of the cementing operation depends on the effective displacement of the drilling fluid by the cement. The efficiency of the displacement step increases as the heavier and more viscous the displacing fluid is than the displaced fluid.
While the density of the cement is an easily controllable property, it is not the same for the rheological properties. The latter can vary considerably under the harsh temperature and pressure conditions encountered in a well. It has been demonstrated that the rheological properties of the standard slurry formulations are not very sensitive to pressure but depend greatly on temperature.
Ordinary slurry viscosity-enhancing agents, such as certain high molecular weight polymers, such as cellulose derivatives, can provide considerable viscosity levels at ambient temperature. However, they lose their effect at temperatures on the order of 100.degree. C. The main reason for this phenomenon is that these products are not sufficiently chemically stable in the presence of high contents of calcium and at high pH levels of the slurries.
The effect of temperature is much more pronounced for the cement than for the drilling fluid. From 25.degree.-120.degree. C., for example, the shear stress at a given shear rate can drop by a factor of 10 for the slurry, whereas for the drilling fluid, it diminishes by a factor of between 2-4. Thus, while at ambient temperature, it is possible to have slurries clearly thicker than the drilling fluid; at 120.degree. C., the cement can become clearly less viscous. This can have serious consequences for the success of the cementing (non-homogeneous setting leading to partial or total loss of the fluid-tight seal).